An olefin resin sheet is excellent in heat resistance, oil resistance and the like and has been used for forming (vacuum forming, pressure forming, etc.) to produce formings, such as various containers, cups, and trays. However, having sharper melt properties than other resins such as polystyrene and polyvinyl chloride, an olefin resin sheet considerably sags when preheated for thermoforming, such as vacuum forming, so that the resulting formings are apt to suffer from forming defects, such as wrinkles, thickness variation, or holes.
It is known that the sag of an olefin resin sheet in thermoforming can be reduced by (1) using a polyblend of a polypropylene resin and a polyethylene resin, which is commonly practiced (see JP-A-52-136247 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-55-108433 and JP-B-63-30951 (the term "JP-B" as used herein means an "examined Japanese patent publication")); (2) adding bismaleimide to a polyolefin (see JP-A-3-52493); (3) using a composition comprising polypropylene, an inorganic filler, and a maleic anhydride-modified or silane-modified polyolefin (see JP-A-51-69553 and JP-A-52-15542); or (4) adding fibrous polytetrafluoroethylene to a polyolefin (see JP-A-8-165358).
In recent years price competition is getting harder in, for example, the food container market, and the proportion of a recycled material in a molding material tends to increase, climbing to about 30 to 50% by weight in a polyolefin molding material. An olefin resin sheet molded from such a molding material with a width as large as about 900 mm or more is liable to sag particularly appreciably. On the other hand, process and quality requirements for containers, which are in conflict with price reduction, are getting stricter. For example, where a sheet is formed into a container with a cover tightly fitted on that is designed to improve hygiene or handling properties, it must be heated for at least double the usual time. This further increases the tendency to sag. Further, the criteria of appearance inspection has also been getting rigid, increasing rejects on account of tiny wrinkles, incorporation of foreign matter of 0.2 mm or smaller, or a slight difference in color.
Under such circumstances, addition of bismaleimide, inorganic filler and a modified polyolefin, or fibrous polytetrafluoroethylene to a polyolefin does not achieve sufficient improvement in view of the cost incurred. That is, the production loss increases due to incorporation of foreign matter during molding into sheeting or insufficient kneading, which is contradictory to the cost reduction. The technique comprising merely blending polyethylene is accompanied by such problems as stones or insufficient dispersion on account of the low melt flow rate.
With the recent increase of frozen food or fast food for microwave oven, vacuum forming containers with a cover fitted on and deep drawn vacuum forming containers which have little residual strain and retain heat resistance or freeze resistance have been widely spreading. Accordingly, it has keenly been demanded to develop a forming sheet stock which involves little production loss or is free from foreign matter incorporated during molding, which is allowed to contain an increased amount of a recycled material, and yet which can have a width as large as 1 m or more to improve the productivity in vacuum forming.